It is well known that cellulose esters are important as commercial plastics and as fibers. In general, cellulose esters are used in plastic applications where hard but clear plastics are required. For example, cellulose esters are used in tool handles, eyeglass frames, toys, toothbrush handles, and the like. All of these applications require a combination of high melting and glass transition temperatures as well as high modulus and good tensile strength. Formulations based on cellulose esters which provide plastic films with low modulus but good tensile strength while maintaining sufficient melting and glass transition temperatures (Tg) to allow thermal processing are generally unknown. Formulations based on cellulose esters which allow thermal extrusion of fibers are also generally unknown.
Because of the high melt temperatures and low melt stability of many of the cellulose esters, plasticizers such as dioctyl adipate or triphenylphosphate are often added to the cellulose ester to lower the melt temperatures during melt processing of the polymer. Although this technique is effective, addition of a monomeric plasticizer often creates secondary problems related to volatile or extractable plasticizers such as dye drip during melt extrusion or long-term dimensional stability (creep) in an object made from the cellulose ester.
The most basic requirement for polymer-polymer miscibility is that the free energy of mixing be negative (.DELTA.G&lt;0). Although on the surface it would seem that polymer-polymer miscibility would be common, in reality there are only a few known miscible binary blends and even fewer known miscible ternary blend systems (Brannock, G. R.; Paul, D. R., Macromolecules, 23, 5240-5250 (1990)). The discovery of miscible binary or ternary blends is very uncommon.
The classical experimental techniques for determining polymer blend miscibility involve the determination of the optical clarity of a film made from the blend, measurement of the appropriate mechanical properties, and measurement of the glass transition temperature by an appropriate thermal analysis technique such as dynamic mechanical thermal analysis (DMTA) or differential scanning calorimetry (DSC). If a blend is miscible, films made from the blend will generally be clear. Likewise, mechanical properties of a blend, such as tensile strength or tangent modulus, are often intermediate between those of the blend components. Furthermore, a miscible amorphous blend will show a single Tg intermediate between that of the component homopolymers while an immiscible or partially miscible blend will show multiple Tg's. In the case of a completely immiscible blend, the Tg's will be those of the homopolymers. For partially miscible blends, the Tg's will be intermediate values corresponding to partially miscible phases rich in one of the components. The variation in binary blend Tg can be modeled by the Fox-Flory equation, Tg.sub.12 =Tg.sub.1 (W.sub.1)+Tg.sub.2 (W.sub.2, where Tg.sub.12 is the Tg of the blend, Tg.sub.1 and Tg.sub.2 are the Tg's of homopolymers, and W.sub.1 and W.sub.2 are the weight percent of each component in the blend. Since the Fox equation does not take into account specific interaction between the blend components the Gordon-Taylor equation, Tg.sub.2 =Tg.sub.1 +kW.sub.2 (Tg.sub.2 -Tg.sub.12)/W.sub.1 ! where k is a constant, is often preferred in blend analysis. For a homogenous, well mixed system, a plot of Tg.sub.12 versus W.sub.2 (Tg.sub.2 -Tg.sub.12)/W.sub.1 will yield a straight line the slope of which is equal to k and the ordinate intercept will be equal to Tg.sub.1. The constant k is often taken as a measure of secondary interactions between the blend components. When k is equal to one, the Gordon-Taylor equation reduces to a simple weight average of the component Tg's.
Miscible blends of cellulose esters and other polymers are generally unknown. The most notable exceptions include the work disclosed by Koleske, et al. (U.S. Pat. No. 3,781,381 (1973)), Bogan and Combs (U.S. Pat. No. 3,668,157 (1972)), Waniczek et al., (U.S. Pat. No. 4,506,045 (1985)), and Wingler et al. (U.S. Pat. No. 4,533,397 (1985)). Koleske et al. reported that blends, formed by solution casting of polycaprolactone and cellulose ester mixtures, are miscible. Later work by Hubbell and Cooper (J. Appl. Polym. Sci., 1977, 21, 3035) demonstrated that cellulose acetate butyrate/polycaprolactone blends are in fact immiscible. Bogan and Combs have reported that block copolymers of polyether-polyesters form miscible blends with some cellulose esters. Critical to the invention of Bogan and Combs was the use of an elastomeric block copolymer; they report that the corresponding homopolymeric elastomers were incompatible with cellulose esters. Waniczek et al., have disclosed that polyester carbonates and polyether carbonates copolymers form miscible blends with many cellulose esters and are useful as thermoplastic resins. Wingler et al. report that contact lenses can be prepared from blends consisting of (A) 97-70% by weight of one or more cellulose esters and (B) 3-30% by weight of an aliphatic polymeric compound having ester moieties, carbonate moieties, or both ester and carbonate moieties in the same polymer chain. The invention of Wingler et al. is limited to aliphatic polymeric compounds; no reference is made to random copolymers consisting of aliphatic diacids, aromatic diacids, and suitable diols or polyols. The invention of Wingler is further limited to cellulose mixed esters having a weight per cent hydroxyl of 1.2% to 1.95% (DS.sub.OH =0.11-0.19 where "DS" or "DS/AGU" refers to the number of substituents per anhydroglucose unit where the maximum DS/AGU is three). The invention of Wingler et al. is also limited to binary miscible blends and by the composition range of the blends (3-30% aliphatic polymeric compound). No reference is made to blends containing an immiscible component where the immiscible component is useful for enhancing properties such as water vapor transmission rates or biodegradability. Immiscible blends of cellulose esters and aromatic polyesters have also been disclosed by Pollock et al. (U.S. Pat. No. 4,770,931 (1988)) which are useful in applications such as paper substitutes.
One time use, disposable items are common. Examples of such disposable articles include items such as infant diapers, incontinence briefs, sanitary napkins, tampons, bed liners, bedpans, bandages, food bags, agricultural compost sheets, and the like. Examples of other disposable items include razor blade handles, toothbrush handles, disposable syringes, fishing lines, fishing nets, packaging, cups, clamshells, and the like. For disposable items, environmental non-persistence is desirable.
Disposable articles are typified by disposable diapers. A disposable diaper typically has a thin, flexible polyethylene film cover, an absorbent filler as the middle layer, and a porous inner liner which is typically nonwoven polypropylene. The diaper construction also requires tabs or tape for fastening the diaper (typically polypropylene) as well as various elastomers and adhesives. Although the absorbent filler is usually biodegradable or easily dispersed in an aqueous environment, currently neither the outer or inner liner nor the other parts such as the tabs or adhesives will degrade from microbial action. Consequently, disposable absorbent materials such as diapers accumulate in landfills and place enormous pressure on waste systems. Other disposable articles such as plastic bags or plastic compost sheets suffer from similar problems.
Numerous studies have demonstrated that cellulose or cellulose derivatives with a low degree of substitution, i.e., less than one, are biodegradable. Cellulose is degraded in the environment by both anaerobic or aerobic microorganisms. Typical endproducts of this microbial degradation include cell biomass, methane(anaerobic only), carbon dioxide, water, and other fermentation products. The ultimate endproducts will depend upon the type of environment as well as the type of microbial population that is present. However, it has been reported that cellulose esters with a DS greater than about one are completely resistant to attack by microorganisms. For example, Stutzenberger and Kahler (J. Appl. Bacteriology, 66, 225 (1986)) have reported that cellulose acetate is extremely recalcitrant to attack by Thermomonospora curvata.
Polyhydroxyalkanoates (PHA), such as polyhydroxybutyrate (PHB), polycaprolactone (PCL), or copolymers of polyhydroxybutyrate and polyhydroxyvalerate (PHBV), have been known for at least twenty years. With the exception of polycaprolactone, they are generally prepared biologically and have been reported to be biodegradable (M. Kunioka et al., Appl. Microbiol. Biotechnol., 30, 569 (1989)).
Polyesters prepared from aliphatic diacids or the corresponding carboxylic ester of lower alcohols and diols have also been reported to be biodegradable. For example, Fields and Rodriguez ("Proceedings of the Third International Biodegradation Symposium", J. M. Sharpley and A. M. Kaplan, Eds., Applied Science, Barking, England, 1976, p. 775) prepared polyesters from C2-C12 diacids coupled with C4-C12 diols and found that many were biodegradable.
Aliphatic polyesters have been used in very few applications mainly because of their low melting points and low glass transition temperatures (generally less than 65.degree. C. and -30.degree. C., respectively). At room temperature, the physical form of many of the aliphatic polyesters is as a thick, viscous liquid. Therefore, aliphatic polyesters are not expected to be generally useful.
On the other hand, aromatic polyesters, such as poly(ethylene terephthalate), poly(cyclohexanedimethanol terephthalate), and poly(ethylene terephthalate-co-isophthalate), have proven to be very useful materials. Aromatic polyesters, however, are generally very resistant to biodegradation (J. E. Potts in "Kirk-Othmer Encyclopedia of Chemical Technology", Suppl. Vol, Wiley-Interscience, New York, 1984, pp. 626-668). Block copolyesters containing both aliphatic and aromatic structures have been prepared and have been shown to be biodegradable. Examples of aliphatic-aromatic block copolyester-ethers include the work of Reed and Gilding (Polymer, 22, 499 (1981)) using poly(ethylene terephthalate)/poly(ethylene oxide) where these block copolymers were studied and found to be biodegradable in vitro. Tokiwa and Suzuki have investigated block copolyesters such as those derived from poly-(caprolactone) and poly(butylene terephthalate) and found them to be degraded by a lipase (J. Appl. Polym. Sci., 26, 441-448 (1981)). Presumably, the biodegradation is dependent upon the aliphatic blocks of the copolyesters; the blocks consisting of aromatic polyester are still resistant to biodegradation. Random aliphatic-aromatic copolyesters have not been investigated in this regard.
While random copolyesters with low levels of aliphatic diacids are known (e.g., Droscher and Horlbeck, Ange. Makromol. Chemie, 128, 203-213(1984)), copolyesters with high levels (&gt;30%) of aliphatic dicarboxylic components are generally unknown. Copolyesters with as much as 40% aliphatic dicarboxylic acid components have been disclosed in adhesive applications; however, these copolyesters adhesives contain at least two dialcohol components in order to achieve the desired adhesive properties (Cox, A., Meyer, M. F., U.S. Pat. No. 4,966,959 (1990)).
There are many references to the preparation of films from polymers such as polyhydroxybutyrate (PHB). Production of films from PHB generally involves solvent casting principally because PHB polymers tend to remain sticky or tacky for a substantial time after the temperature has dropped below the melting point of the PHB. To circumvent this problem, Martini et al. (U.S. Pat. Nos. 4,826,493 and 4,880,592) teach the practice of co-extruding PHB with a thermoplastic that is non-tacky. Such thermoplastics remain as a permanent layer on the PHB film or may be a sacrificial film which is removed following extrusion.
PHB has also been reported to be useful in the preparation of disposable articles. Potts (U.S. Pat. Nos. 4,372,311 and 4,503,098) has disclosed that water soluble polymers such as poly(ethylene oxide) may be coated with biodegradable water insoluble polymers such as PHB. In these inventions, the PHB layer, which is distinct from the water soluble layer, degrades exposing the water soluble layer which will then disperse in an aqueous environment.
There have been other reports of the preparation of a biodegradable barrier film for use in disposable articles. Comerford et al. (U.S. Pat. No. 3,952,347) have disclosed that finely divided biodegradable materials such as cellulose, starch, carbohydrates, and natural gums may be dispersed in a matrix of nonbiodegradable film forming materials which are resistant to solubility in water. Wielicki (U.S. Pat. No. 3,602,225) teaches the use of barrier films made of plasticized regenerated cellulose films. Comerford (U.S. Pat. No. 3,683,917) teaches the use of a cellulosic material coated with a water repellent material.
There exists in the market place the need for thermoplastics which are useful in molding, fiber, and film applications. For these applications, it is desirable that the thermoplastic blend be processable at a low melt temperature and have a high glass transition temperature. These thermoplastics should not contain volatile or extractable plasticizers. Moreover, there is a need in the marketplace for a biodegradable material for use in disposable articles such as diapers, razors, and the like. As an example, unlike films prepared from polymers such as PHB, the material should be amenable to both solvent casting and melt extrusion. In melt extruding this material, coextrusion with other thermoplastics should not be a requirement. The barrier properties of this new biodegradable material should be adequate so that coating with a water insoluble polymer is not required. The new material should disperse completely in the environment and not require coating with a water soluble polymer. The mechanical properties of the material should be such that films of low modulus but of high tensile strength can be prepared.